Organic light-emitting devices, also referred to as organic electroluminescent (EL) devices or as organic internal junction light-emitting devices, contain spaced electrodes separated by an organic light-emitting structure (also referred to as an organic EL medium) which emits light in response to the application of an electrical potential difference across the electrodes. At least one of the electrodes is light-transmissive, and the organic light-emitting structure can have a multi-layer of organic thin films which provide for hole injection and transport from an anode, and for electron injection and transport from a cathode, respectively, with light emission resulting from electron-hole recombination at an internal junction formed at an interface between the hole-transporting and the electron-transporting thin films. As employed herein, the term "thin film" refers to layer thicknesses of less than 1 micrometer with layer thickness of less than about 0.5 micrometer being typical. Examples of organic light-emitting devices containing organic light-emitting structures and cathode constructions formed by thin film deposition techniques are provided by Tang U.S. Pat. No. 4,356,429; VanSlyke et al., U.S. Pat. Nos. 4,539,507 and 4,720,432; and Tang et al., U.S. Pat. No. 4,769,292.
During operation of an organic light-emitting device, the spectral distribution of emitted light (measured in terms of spectral radiance) is related to the electroluminescent properties of the organic thin films used in the device construction. For example, if an organic light-emitting structure includes a layer which contains a light-emitting host material, the emitted light will be dominated by the light emission from the host material.
Tang et al., in the above-cited U.S. Pat. No. 4,769,292 recognized that advantageous performance features of an organic light-emitting device could be obtained if the device included a luminescent zone (or light-emitting layer) of less than 1 micrometer in thickness and comprised of an organic host material capable of sustaining hole-electron recombination, and a small amount of fluorescent material capable of emitting light in response to energy released by hole-electron recombination. The introduction of a fluorescent material into a layer of a light-emitting host material will modify the color of the light emission, and can improve the operational stability of an organic light-emitting device. In analogy to terminology used in the semiconductor industry, fluorescent materials dispersed uniformly at relatively low concentration in light-emitting organic host materials are called "dopants."
As currently practiced, the organic thin films of a light-emitting device are formed by vapor deposition (evaporation or sublimation) in successive deposition steps within a vacuum system which employs a deposition rate control. When a fluorescent dopant is to be uniformly incorporated within an organic light-emitting layer, the light-emitting host material and the fluorescent dopant material are co-deposited from two independently controlled deposition sources. It has been found to be difficult to reliably control the deposition rate of a fluorescent dopant when a desired dopant concentration in the host material of the organic light-emitting layer is at or near a lower end of a dopant concentration range of 10.sup.-3 to about 10 mole percent. The difficulty of reliably controlling the deposition rates of an organic light-emitting host material and of a fluorescent dopant material has been an obstacle in the process of reproducibly fabricating organic electroluminescent devices containing a fluorescent dopant or fluorescent dopants.